Automatic sensitivity control for a mass spectrometer



May 5, 1970 L. WOOD 3,510,647

AUTOMATIC SENSITIVITY CONTROL FOR A MASS SPECTROMETER Filed Sept. 5,1967 MONlTOR woaREF.

SOURCE FUNCTION MHV GEN. s-rAa.

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INVENTOR. LESLIE W000 ATTORNEY-5.

United States Patent U.S. Cl. 250-413 17 Claims ABSTRACT OF THEDISCLOSURE whose gain is varied as a function of monitor collectorcurrent.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to mass spectrometers and, more particularly, to means forautomatically controlling the overall sensitivity of a mass spectrometerto compensate for changes in source sensitivity and sample flow rate.

Description of the prior art In mass spectrometry, a sample is ionizedand then the produced ions are propelled as a beam along an evacuatedion beam tube. In a so-called double focusing instrument, the ions ofthe beam are deflected by both electrostatic and magnetic analyzers.Certain or focused ones of the ions pass through a resolving slit onto acollector. As used herein, the term analyzer means a magnetio analyzeras used in a single focusing instrument, or both an electrostaticanalyzer and a magnetic analyzer, as used in a double focusingspectrometer.

The amount a given ion is deflected by an analyzer a function of itscharacteristic mass and electrical charge, its mass/charge ratio. Theidentity of an ion striking a collector at any given time can bedetermined because, with a given accelerating voltage for propelling theion beam and known analyzer conditions, only ions of a certainmass/charge ratio are focused onto the collector.

In mass spectrometer operation it is common to scan. Most commonly, thisis accomplished by varying the energy applied to the magnetic analyzerso as to deflect differing ones of the ions comprising an ion beam ontoa collector as the analyzer is scanned. This provides a spectrum of thecomponents of the ion beam.

One of thet present day methods of analyzing complex chemical mixturesis first to sepaarte or partially sepaarte the components by means of avapor phase chromatograph, and then to identify the components by meansof a mass spectrometer. The components emerging from the chromatographmay be collected and admitted in turn to the mass spectrometer or thetwo instruments may be directly coupled, so that the separate componentsemerging from the chromatograph pass immediately to the spectrometerinlet. The second method is preferred, because it is faster andminimizes manual handling of samples.

This preferred method presents two problems in mass spectrometry:

(a) It is necessary to scan the mass spectrum during a time when anemergent pulse of gas is emitted by the chromatograph. Since the scanmay occupy an appreciable fraction of the pulse duration, the inletpressure to the mass spectrometer will be continually changing duringthe scan. This change in inlet pressure causes a distorted massspectrum.

(b) In order to produce a spectrum in a very short time, as required formicrocolumns, it is often necessary to use voltage scanning, that is,scanning of the ion accelerating voltage, instead of magnetic scanning.Voltage scanning also distorts the spectrum, because source sensitivitychanges with ion accelerating voltage.

Both of these defects can be considerably reduced by the application ofautomatic sensitivity control to the mass spectrometer, in accordancewith the present invention.

SUMMARY OF THE INVENTION A monitor collector situated between the sourceand analyzer of the mass spectrometer collects a constant fraction ofthe total ion beam emerging from the source. The monitor collectorcurrent is thus a measure of both source sensitivity and inlet gaspressure. The separated ion beams from the analyzer or analyzers of themass spectrometer are collectetd and amplified by means of an electronmultiplier, the gain of which, being a function of the high voltage (HV)applied to it, is controlled by controlling the multiplier high voltage(MHV) as a function of the monitor collector current. Electronmultiplier gain is not a linear function of MHV, but if the output fromthe monitor, or from an amplifier following it, is passed through anappropriate function generator, it can be used to control the mutliplierHV in such a way that the overall sensitivity of the measuring system ofthe spectrometer is substantially independent of both source sensitivityand sample flow rate over a wide range.

Since there is a high capacitance between the dynodes and collector ofan electron multiplier, a continuouslychanging MHV will tend to create adirect current (D.C.) signal in an amplifier following the multiplier.This eflect can be overcome by applying the output of the functiongenerator to the multiplier collector through a suitable capacitorhaving a very high leakage resistance. This creates a DC. signal whichis equal and opposite to the unwanted signal, which is thereforeneutralized.

Assuming for simplicity that multiplier gain versus multiplier highvoltage is a true exponential, then G=e where G is the gain, Vm is theMHV and K is a constant.

Monitor amplifier current (I) is a measure of the combined effects ofsample gas pressure and source sensitivity. For constant sensitivity theproduct IG must be a constant (K This last equation implies that theelectron multiplier high voltage becomes infinite at zero monitorcollector current, which is clearly impracticable. However, initialconditions can be selected such that the multiplier HV is preset to adesired maximum value for monitor currents between zero and somepredetermined value, such as 1% of maximum, after which the multiplierHV falls as monitor collector current rises according to the foregoingequation. Thus, when I=l% max.

so Vm and hence electron multiplier gain have their maximum value. Kthen represents the MHV which gives this multiplier gain, and the gainis reduced in the desired manner by subtracting from K a voltageproportional to the log of the monitor current.

If the monitor current is passed through a high value resistor and theresultant voltage is amplified by means of an amplifier, the output ofthis amplifier is then a voltage which is proportional to the combinedeffects of both source sensitivity and inlet sample pressure. The outputvoltage is then fed to a function generator which, in its simplest form,gives an output voltage which is proportional to the log of the input.However, since log =eo, a log output can have no true zero. The functiongenerator is accordingly designed so that it does not respond to thefirst fraction of the inputsay the first l%but the output isproportional to the log of the input over the remainder of the inputrange. Thus, in the case quoted, the output would be logarithmic over 2decades.

If the particular design of electron multiplier used has a gain versusMHV characteristic which is closely exponential over 2 or 3 decades, andif the MHV is obtained in the usual manner from a voltage stabilizer inwhich the MHV is proportional to a reference voltage, it follows thatthe multiplier gain is proportional to the exponential of the referencevoltage. The output of the function generator is then caused to modifythe reference voltage in such a way that the product of the monitoramplifier output and the multiplier gain is constant, which gives thedesired result.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a combined schematic andblock diagram of a mass spectrometer embodying the invention; and

FIG. 2 is a circuit diagram of a function generator utilized in theembodiment of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, a massspectrometer indicated generally by the numeral includes an ion source12 for providing a positive ion beam 14 to a magnetic analyzer 16 inwhich the ions are separated according to their mass/charge ratios intoindividual ion beams such as which pass individually through a slit 17to an electron multiplier indicated generally by the numeral 18. Themass spectrometer thus far described is conventional and of a type knownin the art.

The mass spectrometer 10 is also provided with a monitor collector 20located between the ion source 12 and the magnetic analyzer 16. Themonitor collector 20 collects a constant fraction of the total ion beam14 emerging from source 12. The current through the monitor collector 20is thus a measure of both the source sensitivity or accelerating voltageand the inlet sample gas pressure. The current through the monitorcollector 20 is amplified by a monitor amplifier 22 and provided as anegative-going signal to a function generator 24. The monitor amplifier22 is conventional in design and is understood to include any dynamicelectronic system for amplifying very small currents. Thus, it may be adirectcoupled amplifier using a very high value of input resistance, orit may be a vibrating reed amplifier in which the input current isconverted to an alternating current by means of a vibrating reedcapacitor before amplification. In either case, the active elements ofthe amplifier may be electron tubes or transistors. Such basic types ofvery small current amplifiers are well known in the art.

The function generator 24, which will be described with reference toFIG. 2, essentially converts a negative-going input signal from themonitor amplifier 22 into a negativegoing output signal, which isproportional to a logarithm of the input signal.

The electron multiplier 18 includes a plurality of dynodes 18D and acollector 18C. Multiplier high voltage (MHV) is applied to the dynodesfrom taps on a voltage divider resistor 26 connected between the outputof an MHV stabilizer 28 and ground. The dynode on which the ion beam 15impinges is at the most negative potential of all the dynodes, which areat progressively decreasing negative potentials as they approach thecollector 180. This is necessary because the output of the electronmultiplier 18 must be near ground potential to simplify feeding anamplifier 30 connected to the collector 18C. The amplifier 30, which issimilar to the monitor amplifier 22 previously described has a highvalue input resistor 36 and output resistor 34 with negative feedback toresistor 36. The output may be read on meter 32 or on a recorder (notshown) driven from the amplifier.

The MHV stabilizer 28 is a conventional, well known device in which theoutput voltage is strictly proportional to a given reference inputvoltage. The output voltage is capable of varying over a wide range, ifthe reference voltage is varied. In the present case, a positive primaryreference potential is provided from a source (not shown) through anadding resistor 38 to the input of the MHV stabilizer 28. A constantfraction of the MHV is compared with the reference potential by means ofthe resistor 38 and a second adding resistor 40 connected across the MHVstabilizer 28. The resulting error signal is amplified and used tocontrol the MHV in such a way that it is proportional to the primaryreference voltage. This technique is well known in the art. The outputof the function generator 24 is also provided through a third addingresistor 42 as an input to the MHV stabilizer 28.

The function generator 24, taking its input from the monitor amplifier22, is designed in such a way that when the monitor amplifier output isless than 1% of its maximum value, the output of the function generatoris zero. When the monitor amplifier output is at its maximum value, thefunction generator output is equal to A of the primary referencevoltage, but is opposite in polarity. If the adding resistor 42 is halfthe value of the adding resistor 38, the effect is to make the effectivereference voltage half the primary reference voltage. Under theseconditions, the MHV is halved and the multiplier gain fal ls two decadesfrom the previously selected maximum gain.

. Since the MHV is negative, a reduction in it is a positive-goingvoltage. Thus, the inherent capacitance existing between the electronmultiplier dynodes 18D and the collector 18C feeding the amplifier 30places a posi tive voltage on the amplifier input which is proportionalto the rate of change of the MHV. However, in order to produce areduction in HMV, the output of the funct1on generator 24 must benegative-going. If then, the output of the function generator is fed tothe input of the amplifier 30 through a capacitor 44, it Will produce anegative voltage at the input of the amplifier 30, which is proportionalto the rate of change of the function generator output voltage and alsoto the capacit ance of the capacitor 44. By suitable choice of the valueof the capacitor 44, the two induced voltages present at the input ofthe amplifier 30 may be made to cancel each other.

The function generator 24, which is shown schematically in FIG. 2, isonly one of a number of devices embodying various forms of logarithmiccircuitry that may be utilized. Such devices are generally quite wellknown in the art, and the function generator shown in FIG. 2

is exemplary only. The particular device shown takes advantage of aproperty of many semi-conductor diodes that the voltage drop across thediode is closely proportional to the logarithm of the current flowingthrough the diode over a wide range of values.

As shown, power for the function generator 24 is provided from analternating current source (not shown), which is connected to a primarywinding 46 of a transformer 48. A secondary winding 50 of thetransformer 48 is connected to opposing input terminals of a full wavebridge rectifier 52. A positive output terminal of the rectifier 52 isconnected to a negative terminal of the rectifier through a seriescombination of a resistor 54 and Zener diodes 56, 58, 60. Positivepotential is supplied to the elements of the generator on a line 62connected to the juncture of the resistor 54 and the diode 56. Thejuncture of the diodes 56, 58 is connected to ground, a maximum negativepotential is supplied on a line 64 from a negative output terminal ofthe rectifier 52, and an intermediate negative potential is suppliedfrom the juncture of the diodes 58, 60 on a line 66. A capacitor 68connected across the output terminals of the rectifier 52 serves tosmooth the output voltage of the rectifier.

The negative-going input signal from the monitor amplifier 22 isprovided through series-connected resistors 70, 72 to the base of an NPNtransistor 74. The juncture of the resistors 70, 72 is connected toground through a capacitor 76. Base bias is provided for the transistor74 through a series-connected resistor 78 and a variable resistor 80connected between the transistor base and the positive potential line62. The emitter of the transistor 74 is connected through a resistor 82to the intermediate potential line 66. The collector of the transistor74 is connected to the positive potential line 62 through a resistor 84,and is also connected directly to the base of an NPN transistor 86.

The base of the transistor 74 is also connected to the cathode of adiode 88. The anode of the diode 88 is connected to the movable arm of apotentiometer 90, which is connected in series with a fixed resistor 92between the positive potential line 62 and ground.

The diode 88 is of the logarithmic response type previously described,wherein the voltage drop across the diode is closely proportional to thelogarithm to the current flowing through the diode. When a negativeinput signal is applied from the monitor amplifier 22, current flowsfrom the positive potential line 62 through the resistor 92, thepotentiometer 90, the diode 88 and the resistors 70, 72. The value ofthat current is determined almost entirely by the resistors 70, 72, sothat the current through the diode is closely proportional to the inputvoltage from the monitor amplifier. Thus, the voltage drop across thediode 88 is closely proportional to the logarithm of the input voltage.When the input 'voltage has a predetermined minimum value, 1% of themaximum value as previously noted, the voltage drop across the diode isbacked ofl by adjusting the movable arm of the potentiometer 90, so thatthere is no signal input to the base of the transistor 74. For anylarger (more negative) input voltage up to the maximum value acceptableby the circuit, the voltage developed across the diode 88 is amplifiedby the circuit comprising the NPN transistor 74, an NPN transistor 86and a PNP transistor 96.

The collector of the transistor 86 is connected directly to the positivepotential line 62. The emitter of that transistor is connected directlyto the base of the transistor 96, and through a resistor 98 to theground. An NPN transistor 94 is connected in series with the transistor96, with the collector of the transistor 94 being connected directly tothe positive potential line 62 and its emitter being connected directlyto the emitter of the transistor 96. The transistor 94, in conjunctionwith resistors 100 102 connected in series between the line 62 andground, the base of the transistor 94 being connected to the juncture ofthose two resistors, provides the correct D.C. level for the emitter oftransistor 96. The output signal of the function generator is developedacross resistance 104, 106 and a potentiometer 108 connected in seriesbetween the collector of the transistor 96 and ground. The collector ofthe transistor 96 is also connected through a resistor 110 to themaximum negative potential line 64 and, through a resistor 114, to anoutput line 1.12. The output line 112 is connected to ground through adiode 116. Output is also provided on a line 118 connected directly tothe collector of the transistor 96. The output line 112 is connected tothe capacitor 44, both of which were referred to in connection with thedescription of FIG. 1.

The collector of the transistor is connected directly to the positivepotential line 62 and its emitter is connected directly to the emitterof the transistor 74 and hence to the intermediate negative potentialline 66 through the resistor 82. The base of the transistor 120 isconnected to the movable arm of the potentiometer 108. Thus, a fixedfraction of the output signal, selected by the position of the arm ofthe potentiometer 108, is fed back to the amplifier circuitry so thatthe overall voltage gain is substantially independent of transistorcharacteristics and can be set to suit the parameters of the particularelectron multiplier utilized and its power supply.

In operation, the variable resistor 80- is set to provide the necessarybase current to the transistor 74 so that this current does not flowthrough the diode 88. The diode 116 connected between the output line112 and ground serves to hold the output of the function generator atzero when the input to the function generator is less than thepredetermined minimum value previously noted. In summary, it is notedthat as the voltage drop across the diode 88 varies logarithmically withrespect to the current flowing through the diode, the output voltagesprovided on the lines 112 and 118 also vary logarithmically with respectto the input voltage from the monitor amplifier 22. It is again pointedout that the invention is in no way limited to the use of the particularfunction generator shown in FIG. 2, and that other devices which operatein essentially the same manner may well be utilized.

Although a preferred embodiment of the invention has been shown anddescribed, it is apparent that many changes and modifications may bemade by one skilled in the art without departing from the true spiritand scope of the invention.

I claim:

1. In a mass spectrometer having a source for emitting an ion beam andan analyzer for separating said ion beam into a plurality of separatedion beams, the improvement comprising:

(a) a monitor collector for collecting a constant fraction of said ionbeam to provide monitor collector current;

(b) an electron multiplier for collecting and amplifying said separatedion beams and having a variable gain;

(c) a source of high voltage for said electron multiplier; and

(d) control means connected in circuit with said source of high voltageand said monitor collector for varying said high voltage to maintainsubstantially constant a product of said monitor collector current andsaid electron multiplier gain.

2. The improvement of claim 1, wherein said control means comprisesfunction generator means for providing an output signal that varies as alogarithmic function of an input signal.

3. The improvement of claim 2, wherein said function generator meansincludes in its input circuitry semiconductor means current throughwhich produces a voltage drop that is substantially proportional to thelogarithm of said current.

4. The improvement of claim 1, wherein said source of high voltagecomprises stabilizer means for providing a high voltage output that isproportional to a reference voltage input.

5. The improvement of claim 1, wherein said source of high voltagecomprises stabilizer means for providing a high voltage output that isproportional to a reference voltage input, and said control meanscomprises function generator means for providing an output signal thatvaries as a logarithmic function of an input signal, said output signalbeing provided to said stabilizer means to modify said reference voltageinput.

6. The improvement of claim 5, wherein said function generator meansincludes in its input circuitry semiconductor means current throughwhich produces a voltage drop that is substantially proportional to thelogarithm of said current.

7. The improvement of claim 1, further including means in circuit withsaid control means and with said electron multiplier to neutralize anyunwanted output signal from said electron multiplier resulting fromvarying said high voltage.

8. The improvement of claim 1, wherein said high voltage is negative.

9. The improvement of claim 8, wherein said electron multipliercollector is substantially at ground potential in the absence of saidmultiplier output signal.

10. In a mass spectrometer having a source for emitting an ion beam andan analyzer for separating said ion beam into a plurality of separatedion beams, the im provement comprising:

(a) a monitor collector for collecting a constant fraction of said ionbeam to provide monitor collector current;

(b) an electron multiplier for collecting and amplifying said separatedion beams and having a variable gain, said multiplier having a pluralityof dynodes and a collector for providing a multiplier output signal;

(c) a source of high voltage for supplying high voltage to said dynodes;

(d) first amplifier means connected to said electron multipliercollector for amplifying said multiplier output signal; (e) secondamplifier means connected to receive said collector current; and (f)function generator means connected to receive said monitor output signaland provide a control signal to said source of high voltage to vary saidhigh voltage supplied to said dynodes to maintain substantially constanta product of said monitor collector current and said electron multipliergain.

11. The improvement of claim 10, wherein said control signal from saidfunction generator means varies as a logarithm of said monitor outputsignal.

12. The improvement of claim 11, wherein said function generator meansincludes in its input circuitry semiconductor means current throughwhich produces a voltage drop that is substantially proportional to thelogarithm of said current.

13. The improvement of claim 10, wherein said source of high voltagecomprises stabilizer means for providing a high voltage output for saiddynodes that is proportional to a reference voltage input, and saidcontrol signal from said function generator means varies as alogarithmic function of said monitor output signal, said control signalbeing provided to said stabilizer means to modify said reference voltageinput.

14. The improvement of claim 13, wherein said function generator meansincludes in its input circuitry semiconductor means current throughwhich produces a voltage drop that is substantially proportional to thelogarithm of said current.

15. The improvement of claim 10, wherein said source of high voltagecomprises stabilizer means for providing a high voltage output for saiddynodes that is proportional to a reference voltage input.

16. The improvement of claim 10, further including means in circuit withsaid function generator means and with said electron multiplier toneutralize any unwanted output signal from said electron multiplierresulting from varying said high voltage.

17. The improvement of claim 10, wherein said function generator meansincludes in its input circuitry semiconductor means current throughwhich produces a voltage drop that is substantially proportional to thelogarithm of said current.

References Cited UNITED STATES PATENTS 2,565,265 8/1951 Peterson 250-2072,659,821 11/ 1953 Hipple. 2,854,583 9/ 1958 Robinson.

RALPH G. NILSON, Primary Examiner A. L. BIRCH, Assistant Examiner US.Cl. X.R. 25 0207 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent NO. 0 1 7 Dated May S, 1970 Inventor(s) Leslie 6 It is certifiedthat: error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Col. 1, line 18, "multiplifier" should be --multiplier--;

line 41. after "analyzer" and before "a" insert --is--: line 54, "thet"should be --the--; line 55, "sepaarte" should be --separate--: line 56,sepaarte should be --separate-.

Col. 2, line 21 "collectetd" should be --collected.

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